Reduced-height scimitar antenna



Jan. 30, 1968 J. GOFF 3,366,963

REDUCED-HEIGHT SCIMITAR ANTENNA Filed Nov. 16, 1964 2 Sheets-Sheet l I 11 l 10 rlllllllllllln INVENTOR. J 0 SEPH 6 OFF ATTORNEY Jan.

Filed Nov. 16, 1964 J. GOFF REDUCED-HEIGHT SCIMITAR ANTENNA 2 Sheets-Sheet 2 VERTICAL HORIZONTAL POLARIZATION POLARIZATION VERTICAL HORIZONTAL VERTICAL POLARIZATION POLARIZATION 10 db CIRCLES INVENTOR,

JOSEPH GOFF 4 TTOR/VE) United States Patent 3,356,963 REDUCED-HEIGHT SCIMETAR ANTENNA Joseph Goff, Baldwin, N.Y., assignor to Sperry Rand Corporation, Great Neck, N.Y., a corporation of Delaware Filed Nov. 16, 1964, Ser. No. 411,446 7 Claims. (Cl. 343741) ABSTRACT OF THE DISCLOSURE An antenna having a scimitar-shaped conductive element whose curved sides have been partially flattened and whose narrow edges are disposed transversely to a conductive ground surface, the conductive element extending from a transmission line feed point at one of its ends to an elongated conductive contact with the conductive surface at the other of its ends and further having a substantially fiat capacitive loading member contacting the flattened outermost edge of the conductive element.

This invention relates to a scimitar-type antenna, and more particularly to a modified form of such an antenna wherein the height of the antenna is considerably reduced without appreciably affectin the desired radiating characteristics of the antenna, thereby allowing this modified antenna to be used in environments where space limitations otherwise would prohibit the use of scimitar antennas.

In recent years considerable interest has been shown in various types of spiral antennas, the type known as a scimitar antenna being one of these that has attracted attention. Briefly described, a scimitar antenna is comprised of a fiat conductive member whose shape generally resembles a scimitar saber. The shaped member is disposed perpendicularly on a conductive ground plane, or surface, with the broad end of the blade conductively secured to the ground plane, and with the narrow end of the blade extending through an aperture in the ground plane and connected to a suitable transmission line. The general class of spiral antennas, and the scimitar antenna, are discussed in an article appearing in Aviation Week, July 14, 1958, pp. 75-82, and the scimitar antenna is described in US. Patent 3,015,101, issued Dec. 26, 1961, to E. M. Turner et al.

In the design of scimitar antennas according to the teachings and disclosures of the above two references, the antenna has a height-to-length ratio that generally falls around .5, the length being measured along the ground plane, and the height being the highest point of the curved portion above the ground plane. When the conventional scimitar antenna is intended for use in an electromagnetic wave system in the lower ranges of the radio frequency spectrum, i.e., the high frequency (HF) and very high frequency (VHF) ranges for example, where the wavelengths are relatively long, the length and height dimensions of the antenna become quite large. When the radio system is to be operated from an aircraft, the antenna would be mounted on the outer surface of the aircraft with the scimitar blade extending transversely outwardly from the surface, and a dielectric radome or covering would enclose the antenna. Because of limitations of available space, and because of aerodynamic considerations, it is necessary that the antenna and radome not extend too far beyond the surface of the aircraft. The scimitar antenna has excellent bandwidth characteristic, and has an omnidirectional, dual-polarization radiation characteristic that make it uniquely suitable for use in certain aircraft radio systems. However, for systems operating at lower frequencies where the dimensions of the antenna become large, the conventional scimitar antenna would have a height that would be excessive for mounting on the surface of an aircraft. Therefore, the conventional scimitar antenna, with its unique characteristics and advantages, would be unavailable for use in the aircraft environment described above.

It therefore is an object of this invention to provide a modified scimitar antenna whose height above the ground plane is significantly reduced without appreciably affecting the desirable electrical and radiating characteristics of the scimitar type antenna.

In accordance with the present invention, the height of a scimitar antenna may be significantly reduced by eliminating the outermost portion of the curved surface that is furthest removed from the ground plane, by altering the shape of the innermost curved surface, and by adding a thin, broad conductive member that is disposed transverse ly to the remaining reducedheight portion of the curved surface and extends lengthwise in conductive contact with at least a portion of the outermost edge of the reducedheight portion of the antenna. The above-described structure is mounted on a conductive ground surface in the usual manner, i.e., the broad end portion of the scimitar blade is conductively secured to the ground surface and the oppositely positioned narrow end passes through the ground surface and is connected to a transmission line. The thin, broad conductive member that is added at the top of the curved surface has an overall length slightly less than the length of the modified scimitar curved surface and provides a capacitive loading effect which electrically compensates for the outermost portion of the scimitar shape that was eliminated. The radiation patterns, bandwidth and electrical characteristics of the modified scimitar antenna are substantially the same as those of the much higher, conventional scimitar antenna.

The invention will be described by referring to the accompanying drawings wherein:

FIG. 1 is a perspective view of a modified scimitar antenna constructed in accordance with the present invention;

FIG. 2 is a sectional view of the antenna of FIG. 1 taken at section 2-2 of FIG. 1;

FIG. 3 is an illustration showing the modified scimitar shape of the present invention as compared with the conventional scimitar shape of the prior art antennas; and

FIGS. 4a and 4b show a series of antenna patterns that were made from radiation measurements of an antenna of this invention.

Referring now in detail to FIGS. 1 and 2, the reducedheight scimitar antenna of this invention is comprised of a conductive ground surface 10, which in a practical application might be the outer surface or skin of an aircraft. The thin curved antenna element 18 is made of a conductive material that extends transversely from conductive ground surface 10 and the broad edge 13 on its right side is secured to conductive ground surface 10 to provide a'conductive contact therebetween. The narrow tip 14 at the left end of curved element 11 serves as a feed point for the antenna and is adapted to be connected through aperture 15 to a suitable transmission line that extends from the bottom surface of ground surface 10. A thin flat conductive member 20 is secured in conductive contact along the top outermost edge of curved antenna element 11. Conductive member 20 is everywhere transverse to the curved element 11 and is parallel to conductive ground surface 10 throughout a major portion of its length, being curved only at its left end to achieve conductive contact with the leading edge of the curved antenna element 11.

The outermost edge 22 of curved element 11 defines a curve which departs from the equiangular spiral curve of scimitar antennas of the prior art. This feature is illustrated in FIG. 3 wherein the curve S, represented by the broken line, is the curve of an equiangular spiral that is used to define the outermost edge of conventional scimitar antennas. Because a curve of this type would result in an antenna whose height is too great for practical use in the lower regions of the radio frequency spectrum, the height of the antenna of this invention has been reduced and its outermost edge 22 is defined by the curve S illustrated by the solid line. In order to maintain the desirable characteristics of the scimitar antenna, such as extremely broad bandwidth, and omnidirectional, dual-polarization radiating characteristics, means must be provided to electrically compensate for that portion of the scimitar curve that was removed, i.e., the area in FIG. 3 that lies between the eurves S and S. In accordance with this invention, I provide this compensation by means of the thin conductive member 20 which functions as a capacitive loading member. The effect of the capacitive conductive member 20 on the operation of the antenna has not been completely analyzed in a rigorous way, and to date, the design of antennas of this type has been largely empirical in nature. It is known, however, that as more of the conventional scimitar shape is removed in order to reduce the height of the antenna, the capacitive conductive member 20 must increase in dimensions to provide additional capacitive loading.

In the conventional scimitar antennas of the prior art, the ratio of the height of the antenna over the ground surface to the length of the antenna measured along the ground surface generally has been around .5. I have found that I can successfully reduce this ratio to a figure at least as low as .111 by employing the concepts of this invention, and the resulting electrical and radiating characteristics are not significantly changed from those of the conventional scimitar antenna.

The curve of the innermost edge 23 of the antenna of FIG. 1 also has been flattened relative to an equiangular spiral curve that commonly is employed in conventional scimitar antennas. The length of this innermost edge 23 determines the low frequency cut off of the antenna, and must be designed with this characteristic in mind. In practice, the shapes of the outermost and innermost curved edges 22 and 23 that were required to provide desired optimum antenna characteristics were determined through successive trials of several different curved elements 13. An antenna constructed substantially as illustrated in FIG. 1 and intended for operation in the L and S bands of the microwave frequency spectrum had the following approximate dimensions:

Length of broad end of curved antenna element 11 0.90 W1dth of broad conductive loading element 20 .97 Length of broad conductive loading element 20 4.0

Thickness of conductive material of members 11 and 20 .04

In practice, the structure illustrated in FIG. 1 most likely would be enclosed within a dielectric radome so that the aerodynamic properties of the structure would not be of importance. If the antenna were not enclosed within a protected radome, the flat conductive loading member 20 may be shaped so as to have desired aerodynamic properties.

FIG. 4a illustrates a series of measured radiation patterns of an antenna constructed in the manner shown in FIG. 1, but having a different height-to-length ratio of .16 and intended for operation in the ultra high frequency portion of the radio frequency spectrum. The 0 and characters, respectively, refer to vertical and horizontal angles, as illustrated in the reference axis diagram of FIG. 4b. As may be seen, the antenna operates over an exceedingly wide bandwith to provide omnidirectional, dual-polarization radiation patterns that are commensu- 4 rate with those obtained from the conventional equiangular spiral scimitar antenna.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made withoutdeparting from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

1. A reduced-height scimitar antenna comprising,

a conductive ground surface,

a curved conductive element disposed transversely to said ground surface and extending from a transmission line feed point at one of its ends to an elongated conductive contact with said ground surface at the other of its ends,

conductive means disposed along the outermost edge of said curved element for presenting a capacitive loading effect for said antenna, and

said curved conductive element having the shape of a scirnitar whose curved sides have been flattened over a region between said ends.

2. A reduced-height scimitar antenna comprising,

a conductive ground surface,

a curved conductive element disposed transversely to said ground surface and extending from a transmission line feed point at one end to an elongated conductive contact with said ground surface at its other end,

a conductive member extending in conductive contact along at least a portion of the outermost edge of said curved element and extending transversely on both sides ofsaid curved element, and

said curved conductive element having the shape of a scimitar whose curved sides have been flattened over a region between said ends.

3. A reduced-height scimitar antenna comprising,

a conductive ground surface,

a curved conductive element disposed transversely to said ground surface and extending from a transmission line feed point at one end to an elongated conductive contact with said ground surface at its other end,

the outermost edge of said curved conductive element having a height above said ground surface that is less than the height of a spiral curve that begins at said one end of the element and passes through the other end of the element,

a thin conductive member disposed transversely to and in conductive contact with at least a portion of the outermost edge of said curved element, and

said curved conductive element having the shape of a scimitar whose curved sides have been flattened over a region between said ends.

4. The combination claimed in claim 3 wherein said thin transversely disposed conductive member is disposed symmetrically with respect to the outermost edge of said conductive element and is substantially parallel to said ground surface over a portion of its length.

5. An electromagnetic wave antenna comprising,

a conductive ground surface,

a thin conductive member disposed on said ground surface and extending transversely outwardly therefrom,

said conductive member having the shape of a scimitar whose curved sides have been flattened over a region between said ends,

one end of said conductive member being in elongated conductive contact with said ground surface and the other end of the conductive member being adapted to be coupled through an aperture in the ground surface to an electromagnetic wave transmission line,

a thin conductive loading member having a Width considerably greater than the thickness of said conductive member disposed transversely to said conductive member and extending longitudinally in conductive contact with at least a portion of the outermost edge of said conductive member.

6. A reduced-height scimitar antenna comprising,

a conductive ground surface,

a thin, outwardly curved conductive element lying in a plane disposed transversely to said ground surface and extending from a transmission line feed point at one of its ends to an elongated conductive contact with said ground surface at its other end,

the outermost edge of said curved conductive element having a height above said ground surface that is less than the maximum height of a spiral curve that begins at said one end of the element and passes through the other end of the element,

conductive means disposed along the outermost edge of said curved element for presenting a capacitive loading effect for said antenna, and

said curved conductive element having the shape of a scimitar Whose curved sides have been flattened over a region between said ends.

7. A reduced-height scimitar antenna comprising,

a conductive ground surface,

a thin, outwardly curved conductive element lying in a plane disposed transversely to said ground surface and extending from a transmission line feed point at one of its ends to an elongated conductive contact with said ground surface at its other end,

the outermost edge of said curved conductive element having a height above said ground surface that is less than the maximum height of a spiral curve that begins at said one end of the element and passes through the other end of the element, and

a thin conductive member extended longitudinally in conductive contact along at least a portion of the outermost edge of said curved element and extending transversely on both sides of said curved element.

References Cited UNITED STATES PATENTS 2,566,491 9/1951 Hills 343-828 X 2,575,377 11/1951 Wohl 343-843 2,615,134 10/1952 Carter 343848 X 2,644,089 6/1953 Bliss 343830 X 3,241,148 3/1966 Lechtreck 343895 HERMAN KARL SAALBACH, Primary Examiner.

WILLIAM H. PUNTER, Examiner. 

